Been abroad twice – once with softbag, once with hardbox…going by both experiences I’d say softbag every time.
Soft bag was brand new and apart from a single scuff in the bottom corner, it was still looking immaculate as it came out at either end of the trip (Edinburgh to Austria and back again). Only damage was a slight bend in the rotor as I put the wheel in the wrong way round when packing.
Hard case – trip was Edinburgh to southern Spain and back again – both done on cheaper airlines – my box got dropped from the plane to the ground unloading in Edinburgh – lost brake pads and the box was irrepairably damaged – huge hole in it and if the loose stuff wasn’t tied round the frame, I’d have lost more than just the disc pads. Only loose stuff was what wasn’t attached to the bike – pedals and seat.
No damage on way out (other than 1 lock being twisted, but again I put that down the it being a bit wobbly and probably just knocked) but way back was worrying after seeing the damage.
On that basis I think the baggage handlers took extra care with the soft case rather than the hard case incident…so I’d say go soft case!